This application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2004-004346 filed in Japan on Jan. 9, 2004, the entire contents of which are hereby incorporated by reference.
The present invention relates to image forming apparatuses such as scanner devices, copying devices, facsimile devices, and compound machines that incorporate any of these devices, and methods for controlling fixing mechanism portions.
Conventionally, with image forming apparatuses such as scanner devices, copying devices, facsimile devices, and compound machines that incorporate any of these devices, a sheet of recording paper on which a toner image has been transferred is passed through a fixing mechanism portion and the toner image is thermally fixed onto the sheet of recording paper by the fixing mechanism portion. The fixing mechanism portion is provided with a pair of roller members arranged in opposition to each other and at least one of these members is constituted as a heating roller that acts as a heat source for fixing. That is, the toner image is thermally fixed onto the sheet of recording paper by transporting the sheet of recording paper while it is supported sandwiched between the pair of roller members.
In this regard, with this kind of fixing mechanism portion, it has been proposed that the warming up time of the device can be shortened and its energy efficiency improved without increasing the electrical power consumption (power supply) of the device by making the roller body thinner to reduce its thermal capacity.
However, when using a roller body that has been made thinner in this way, the roller body's thermal transferability in the axial direction is reduced due to being made thinner. For this reason, it becomes difficult to keep the entire roller body at a uniform temperature. For example, when a sheet of recording paper of a size smaller than the roller body's heating range has passed through, in contrast to the passed-through portion of the sheet of recording paper where heat has been absorbed by the sheet of recording paper, since heat is not absorbed at the portion where the sheet of recording paper does not pass through, an excessive rise in the roller temperature (hereafter, “irregular temperature rise at portions un-passed by paper”) occurs at this portion. When a sheet of recording paper larger than the above-mentioned size is passed through under conditions in which such an irregular temperature rise at portions un-passed by paper has occurred, risks are posed such as excessive fixing at the portion of the excessive temperature rise, changes to the glossiness of the toner on the sheet of recording paper, and excessively fixed portions causing high-temperature offset such that toner adheres to the heating roller.
In order to avoid this irregular temperature rise at portions un-passed by paper, conventional fixing mechanism portions have a plurality of heaters of different heating ranges arranged inside the roller body, and heaters to which electricity is to be supplied are selected according to the size of the sheet of recording paper to be passed through (for example, see JP 2003-177627A).
FIGS. 8 and 9 show the internal structure of a conventional fixing mechanism portion 39 and an overall configuration of a heating roller and a configuration of a control circuit therein.
As shown in these drawings, a heating roller 39a is provided with a roller body 391 as a fixing member, halogen heaters 392 as heating means for heating the roller body 391, temperature sensors 393A and 393B constituted by a temperature detection means for detecting a surface temperature of the roller body 391, a control circuit 540, and a pressure roller 39b arranged in opposition so as to be opposing the heating roller 39a. 
The halogen heaters 392 are arranged inside the roller body 391 and are provided with a main heater 392a positioned as a heater aligned with a center reference of a sheet of recording paper in a central portion of the roller axial direction and sub-heaters 392b arranged on both sides on the main heater 392a in the axial direction. The main heater 392a is positioned below the roller axis. On the other hand, the sub-heaters 392b are positioned above the roller axis. The main heater 392a and the sub-heaters 392b are constituted by a filament F accommodated inside a glass tube G, and the filaments F are formed such that portions corresponding to areas to be heated by the heaters 392a and 392b serve as heat-generating locations. Infrared rays are irradiated to achieve a predetermined thermal distribution by applying electricity to the filaments F from the control circuit 540 and the inner surface of the heating roller 391 becomes heated. Furthermore, the main heater 392a and the sub-heaters 392b are respectively and independently temperature-controlled by the control circuit 540.
The roller body 391 is heated by the halogen heaters 392 (the main heater 392a and the sub-heater portion 392b) to a predetermined temperature (for example, 200° C.) and thus heats a sheet of recording paper P, which is a recording medium, that passes through a nip area K (nip area K between the heating roller 39a and the pressure roller 39b) of the fixing mechanism portion 39. Furthermore, the heating roller 39a is provided with a core 39a1, which is the body thereof, and a mold release layer 39a2 formed on an outer surface of the core 39a1 to prevent the toner on the sheet of recording paper from offsetting.
A ferrous material such as iron or stainless steel for example, or an alloy of these, is used for the core 39a1. It is also possible to use a metal such as aluminum or copper. For the mold release layer 39a2, a fluorocarbon resin such PFA (a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether) and PTFE (polytetrafluoroethylene), silicone rubber, or fluorocarbon rubber is used.
The pressure roller 39b is structured such that it has a heat-resistant elastic material layer 39b2 such as silicone rubber around the outer surface of the core 39b1 of iron, stainless steel, or aluminum, or the like. A mold release layer may be formed with the same fluorocarbon resin as in the case of the heating roller 39a on the surface of the heat-resistant elastic material layer 39b2 of the pressure roller 39b. It should be noted that the pressure roller 39b is configured such that it is pressed against the heating roller 39a with a force of approximately 200 N by an elastic member such as a spring not shown in the drawings, and in this way the nip area K is formed with a predetermined width between the pressure roller 39b and the heating roller 39a. 
On the other hand, the control circuit 540 is configured so as to control the heating roller 39a at a fixing temperature (200° C.) with a direct heating method using the halogen heaters 392. That is, the control circuit 540 is provided with drivers 541 that supply electricity to the heaters 392a and 392b, a driver 544 that rotationally drives the roller body 391, a CPU 542 that controls the drivers, and input circuits 543 that receive detection signals from temperature sensors 533A and 533B, and is configured to maintain the surface temperature of the heating roller 39a at the fixing temperature by alternating the state of power supply to the heaters 392a and 392b based on the detection signals from the temperature sensors 533A and 533B. Specifically, the state of the power supply to the main heater 392a is alternately based on a temperature detection signal from the main temperature sensor 533A positioned in the range heated by the main heater 392a. On the other hand, the state of the power supply to the sub-heaters 392b is alternately based on a temperature detection signal from the sub temperature sensor 533B positioned in the range heated by the sub-heaters 392b. 
Then, while electricity is supplied to only the main heater 392a when a small-size sheet is passing through, electricity is supplied to both the main heater 392a and the sub-heaters 392b when a large-size sheet is passing through. In this way, it is possible to heat only the area on the roller body 391 on which the sheet of recording paper passes through.
In this regard, with such power supply control for the main heater 392a and the sub-heaters 392b, although it is possible that only the main heater 392a is supplied electricity (ON) while the sub-heaters 392b are not supplied electricity (OFF) when a small-size sheet is passing through, if the sub-heaters 392b are completely turned OFF, the temperature drops extremely at the axial-direction end portions of the heating roller 39a, and then, even when electricity supply (ON) to the sub-heaters 392b is started in order for a large-size sheet to be subsequently passed through, there is a problem in that the temperature at the axial-direction end portions of the heating roller 39a does not rise rapidly and a fixing deficiency occurs due to an insufficient amount of heat. For this reason, with conventional image forming apparatuses, the sub-heaters 329b are not turned OFF completely even when a small-size sheet is being passed through, and a standby heating is performed such that the temperature at the axial-direction end portions of the heating roller 39a is maintained to a certain extent by intermittently alternating the ON and OFF control. Specifically, if full electricity supply is given as 100%, then electricity supply of 30% is carried out with ON-OFF control. In this way, the above-mentioned fixing deficiency problem is solved when transitioning to printing of a large-size sheet.
On the other hand, control is performed by carrying out ON-OFF control of the sub-heaters 329b when printing a small-size sheet such that the central portion of the heating roller 39a to which the small-size sheet is to be passed through becomes a set fixing temperature in consideration of such factors as heat conduction to the sheet of recording paper, and the temperature at the axial-direction end portions of the heating roller 39a is raised to the temperature of standby heating when continuing to successively print small-size sheets since the sheets of recording paper do not pass through.
FIG. 10 shows an example of temperature distribution in the axial direction of the heating roller 39a. The curved line shown as a solid line in this drawing represents the temperature distribution when a small-size sheet has passed through and the curved line shown as a broken line represents the temperature distribution when a large-size sheet has passed through.
As shown in FIG. 10, when large-size sheets are passing through, the entire axial-direction length of the heating roller 39a is controlled at approximately the set fixing temperature no matter how many sheets are printed, but when small-size sheets are passing through, although control of the set fixing temperature is achieved at the beginning of printing, the temperature at the axial-direction end portions of the heating roller 39a continues to be raised by the preheating amount and, in accordance with the increasing number of printed sheets, the temperature becomes higher than the set fixing temperature (shown by the numerical symbol 81 in this drawing). As a result of measurements with an actual device, it was found that a temperature gap T at this time was in the range of 30° C. to 40° C.
Further still, a discrepancy in the thermal expansion of the pressure roller 39b itself, which is arranged in opposition to and pressed against the heating roller 39a, was caused due to the occurrence of the temperature gap T, and the diameter itself of the pressure roller 39b was different in the axial direction at a central area (paper pass-through area) and end areas (paper non-pass-through areas) due to this discrepancy in thermal expansion. As shown in FIG. 11, the difference in roller diameter is such that L1 (paper non-pass-through areas)>L2 (paper pass-through area) and the diameter of the paper non-pass-through areas becomes larger than the diameter of the paper pass-through area. In FIG. 11, the L2 portion is the portion at which small-size sheets pass through.
Incidentally, in recent years, an increasing number of image forming apparatuses employ a buildup system in order to achieve device compactness and a reduction in the area occupied by the device. That is, there are many formations in which there is a paper-supply portion for storing and transporting sheets of recording paper at the lowest part of the device, an image-forming portion directly above the paper-supply portion, and a document reading portion at the highest portion part of the device.
In such devices, a sheet of recording paper is transported substantially vertically, and at this time the transport force of the sheet of recording paper is subject to the influence of gravity. For example, when the sheet of recording paper passes through the transfer mechanism portion and is transported to the fixing mechanism portion, the sheet of recording paper naturally has an inclination to fall in the direction of the transfer mechanism portion due to the influence of gravity. For this reason, ordinarily it is usual for the peripheral roller speed of the fixing mechanism portion (paper transport speed) to be slightly faster than the speed at which the sheet of recording paper passes through the transfer mechanism portion (peripheral roller speed of transfer mechanism portion), and the speed ratio thereof becomes [(speed at which sheet of recording paper passes through transfer mechanism portion):(speed at which sheet of recording paper passes through fixing mechanism portion)=1.0:(1.02 to 1.005)].
With devices that transport the sheets of recording paper substantially vertically in this way, when the paper width closely resembles the approximate total length of the fixing rollers (the heating roller 39a and the pressure roller 39b) of the fixing mechanism portion by which the sheets of recording paper are carried, there is no discrepancy in thermal expansion such as that described above at the pressure roller 39b, and therefore the fixing and transfer processes can be carried out normally with the above-mentioned speed ratio.
However, when the width of the paper to be transported is that of paper that is narrow (such as for lengthwise transport of a small-size sheet) with respect to the fixing rollers (the heating roller 39a and the pressure roller 39b), temperature unevenness occurs as described above at the central area (paper pass-through area) of the fixing rollers (the heating roller 39a and the pressure roller 39b) at which the sheets of recording paper pass through and the side areas (paper non-pass-through areas) at which the sheets of recording paper do not pass through, and a discrepancy occurs in the thermal expansion of the pressure roller 39b due to this temperature unevenness such that the pressure roller diameters of the paper pass-through area and the paper non-pass-through areas become different as shown in FIG. 11. Since the paper transport speed of the fixing rollers is determined by the peripheral roller speed of the large-diameter non-pass-through areas of the pressure roller 39b, the relative speed ratio changes with respect to the paper transport speed in the transfer process. Specifically, the paper transport speed in the fixing process becomes gradually faster with respect to the paper transport speed in the transfer process in accordance with the thermal expansion of the paper non-pass-through areas of the pressure roller 39b. This disparity becomes conspicuous during successive printing.
The present inventors conducted tests regarding the extent of change in paper transport speeds during the fixing process in successive printing. In these tests, the dimensional error (magnification change) of documents and printing was measured in respective locations at a leading edge area, a central area, and a trailing edge area of sheets of recording paper when 50 sheets were successively printed in the respective cases of when A4-size sheets of recording paper were inserted sideways with respect to the paper transport direction (so called “sideways transport”) and when the sheets were inserted lengthwise (so called “lengthwise transport”). The results thereof are shown in FIG. 12. FIG. 12A is the test results for when A4-size sheets of recording paper were transported sideways and FIG. 12B is the test results for when A4-size sheets of recording paper where transported lengthwise.
The case of sideways transport of A4-size sheets of recording paper corresponds to the passage of paper for large-size sheets and, as shown by the broken line in FIG. 10, the entire axial-direction length of the heating roller 39a is controlled at approximately the set fixing temperature no matter how many sheets were printed. Accordingly, there is no temperature unevenness of the heating roller 39a and no discrepancy in thermal expansion occurs in the pressure roller 39b that is arranged in opposition to it. For this reason, as shown in FIG. 12A, dimensional error (magnification change) of the document and the printing is not related to the number of printed sheets and the difference between the leading edge area and the trailing edge area of the sheets of recording paper is kept approximately constant.
In contrast to this, the case of lengthwise transport of A4-size sheets of recording paper corresponds to the passage of paper for so-called small-size sheets and, as shown by the solid line in FIG. 10, temperature unevenness occurs at the paper pass-through area and the paper non-pass-through areas such that, as shown in FIG. 11, the diameter of the pressure roller 39b is different at the paper pass-through area and the paper non-pass-through areas. The difference in the diameter of the pressure roller 39b increases according to the increase in the number of sheets printed. As a result of this, as shown in 12B, although the dimensional error at the leading edge area of the sheets of recording paper is not related to the number of sheets printed and is maintained at approximately 98.8%, this changes by approximately 0.5% from 99.3% to 98.8% at the trailing edge area of the sheets of recording paper according to the increase in the number of sheets printed. And it was found that this variation in dimensional error was a cause of transfer misalignment at the trailing edge area of the sheets of recording paper.
That is to say, when a sheet of recording paper being transported has a length that substantially spans the transfer roller and the fixing roller, the printing at the leading edge area of the sheet of recording paper is controlled according to the speed of the transfer roller and the printing of the trailing edge area is controlled according to the speed of the fixing roller, but in the case of successive printing, since the transport speed of the fixing roller gradually becomes faster in comparison to the transport speed of the transfer roller with each increase in the number of sheets printed, the trailing edge area (transfer side) of the sheets of recording paper becomes strongly pulled on by the fixing roller and transfer misalignment occurs at the trailing edge area side of the sheets of recording paper.